Micro-electromechanical systems (MEMS) technology allows the manufacture of small (microns to hundreds of micron) electromechanical components using the precision manufacturing techniques of microelectronics. The term often used to describe MEMS manufacturing process is micromachining. Prime examples of MEMS devices on the market include pressure sensors and accelerometers. See L. Sprangler and C. J. Kemp, ISAAC-integrated silicon automotive accelerometer, in Tech. Dig. 8th Int. Conf. On Solid-State Sensors and Actuators (Transducers '95), Stockholm, June 1995, pp. 585–588. See also W. S. Czarnocki and J. P. Schuster. 1995. “Robust, Modular, Integrated Pressure Sensor,” Proc 7th Intl Cong on Sensors, Nuremberg, Germany.
These devices typically consist of thin membranes and beams micromachined from films that are deposited or laminated on a substrate or, etched from the substrate itself. These micro-machined films ultimately serve as the mechanical structure and/or electrical connection. In the case where the film is deposited or laminated, a sacrificial layer is deposited first and then patterned using photolithography prior to film deposition or lamination. Patterning creates regions on the substrate that are free of the sacrificial layer and, thus the film can be deposited directly onto the substrate, i.e. anchored to the substrate. After the film has been micro-machined to a desired structure, the portion of the structure that is not anchored to the substrate is physically separated or disconnected from the substrate by removing the sacrificial layer underneath the micromachined structure.
One major difficulty often encountered in micromachining is the control of film stress. Film stress is the residual stress that is present in the film after formation. For the case of tensile film stress, the resulting micromachined structure will also be in tensile stress unless the micromachined structure is designed to strain and relieve the stress. Micro-machined cantilevers are a typical example of a structure that can relieve the residual stress.
For a MEMS microphone (condenser microphone), the part of the micromachined structure that actuates with an acoustic signal is the diaphragm. Stress on the diaphragm has a direct effect on the sensitivity of the microphone. Tensile stress severely decreases the mechanical compliance of a microphone diaphragm. The following idealized formula shows that decreasing the mechanical compliance decreases the sensitivity of a capacitor microphone:
  Sens  =                    C        MS            ⁢      SE              x      0      
Here, CMS is the mechanical compliance in meters per Newton; S is the area; E is the bias; x0 is the distance between the microphone diaphragm and back plate; and Sens is the open circuit sensitivity of the microphone. It is clear, that in order to fabricate MEMS microphones whose sensitivity is minimally affected by the film stress, the diaphragm must be designed such that film stress minimally affects its mechanical compliance. One must keep in mind that the diaphragm is fabricated from the very film which holds the residual stress.
One method of minimizing the effect of film stress is the free plate scheme. See Loeppert et al., Miniature Silicon Condenser Microphone, U.S. Pat. No. 5,490,220 and PCT application 01/25184, filed Aug. 10, 2001 (claiming priority from U.S. Ser. Nos. 09/637,401 and 09/910,110). In this method, the diaphragm is largely free with the exception of a narrow arm or arms. The function of the narrow arm is simply to provide an electrical connection to the diaphragm. This way the mechanically free diaphragm is allowed to strain and release the residual stress. Since the diaphragm is not rigidly attached to the substrate, it is necessary to mechanically confine the diaphragm on the substrate to prevent the diaphragm from detaching completely while handling. For the free plate design, the diaphragm hovers over an acoustic port that has been etched into the substrate (the substrate opening, which serves as the acoustic port, is smaller than the diaphragm diameter). In the free plate design, the back plate covers the diaphragm and provides the necessary confinement. Thus the diaphragm is confined by the substrate and the back plate located on either side of the diaphragm.
Implementing a microphone diaphragm design such as the free plate design is possible by using a fabrication process that is capable of depositing many conformal layers of thin film (See the above referenced PCT application).
In some microphone implementations, it is desirable to place the backplate between the diaphragm and the substrate. In other devices, there may not be a backplate at all. In these situations, there is insufficient constraints to keep the free plate diaphragm from being pulled away from the substrate and being damaged during processing or when handling a completed device. This invention describes a means to protect the diaphragm by constraining its out of plane travel yet which will strain to relieve in-plane stresses.